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Статья Paper
Porphyrazines with Annulated Chalcogen-Containing Heterocycle: Study of Spectral-Luminescent Properties and Quantum-Chemical Calculations of the Excited Electronic States®
Konstantin N. Solovyov,a@ Pavel A. Stuzhin,b Valery A. Kuzmitsky,a,c Dmitry I. Volkovich,3 Valery N. Knyukshto,3 Elena A. Borisevich,3 and Anwar Ul-Haqueb
a B. I. Stepanov Institute of Physics, National Academy of Science of Belarus', Pr-t Nezavisimosti, 68, Minsk, 220072, Belarus'
b Ivanovo State University of Chemical Technology, Friedrich Engels Pr-t, 7, Ivanovo, 153000, Russia c Institute for Command-Engineer of the Ministry of Emergency Situations of the Republic of Belarus' @Corresponding author E-mail: solovyov@imaph.bas-net.by
Joint experimental and theoretical study of spectral-luminescent properties and electronic structure of the molecules of new phthalocyanine analogues - derivatives of octaphenylporphyrazine and tetrakis(tert-butyl)phthalocyanine carrying an annulated 1,2,5-thiadiazole or 1,2,5-selenadiazole cycle instead of two phenyl groups or a benzene ring - has been performed. The annulation of a five-membered heterocycle results in bathochromic shifts of the Qx band and Soret band; the Qy - Qx(S2 -S1) interval increases and the fluorescence spectra become mirror-symmetrical to the absorption spectra in the region of the G^Qx transition. Replacement of a benzene ring in the tetrakis(tert-butyl)phthalocyanine molecule by the 1,2,5-selenadiazole cycle lowers the fluorescence quantum yield from 0.77 to 0.14 which may be accounted for by the internal heavy-atom effect as well as by intramolecular charge transfer. The fluorescence quantum yield of octaphenylporphyrazine is very low, but for the derivatives it is comparatively high, tyF=0.06, i.e. the fluorescence is not quenched but enhanced when the heterocycle is annulated. The quantum-chemical calculations of the ground and excited states of the original structures of phthalocyanine, octaphenylporphyrazine, and their derivatives with the annulated thiadiazole cycle have been performed. It is shown that the chalcogen substituted compounds exist as isomers with the NH - NH axis passing through the pyrrole rings devoid of the heterocycle. The calculations based on the INDO/Sm method give reasonable interpretation of the experimental absorption spectra. The description of the properties of the lowest excited Q states is sufficiently goodfor the phenyl substituted compounds. At the same time, for the Soret band region it is rather difficult to achieve agreement with experiment. Specifically, the energies of the G^B and G^N transitions and the relation between their intensities strongly depends on the values of the dihedral angle made by phenyl ring and the macrocycle.
Keywords: Porphyrazines, 1,2,5-thia- and 1,2,5-selenadiazole, spectral-luminescent properties, quantum-chemical calculations.
Introduction
The substitution of four benzene rings in the phalocyanine molecules can change their physico-chemical properties and even allow to manage them. Annulation of five- and seven-membered heterocycles to the porphyrazine (tetraazaporphine) macrocycle, H2PA, is an important modification approach in the synthesis of phalocyanine analogues. Study of low-symmetry substituted compounds having from 1 to 3 annulated heterocycles is very important for understanding of the nature of property changes.
In the current paper we report the results of the spectral-luminescence study of new compounds of this type: hexaphenyl(1,2,5-thiadiazolo)porphyrazine (abbreviated
®This paper is dedicated to Professor Boris Dmitrievich Berezin and was originally submitted for the special issue on the occasion of his 80th Birthday (Macroheterocycles 2009, Vol. 2, N. 2).
as H2{SN2}PAPh6), its Se-analogue (H2{SeN2}PAPh6), and also benzo-fused analogue tris(p-tert-butylbenzo) (1,2,5-selenadiazolo)porphyrazine (H2{SeN2}PA(BztBu)3). Here PA is porphyrazine dianion, Ph - phenyl radical, Bz -fused benzene ring, tBu - tert-butyl group. Chart 1 shows the structural formulae of the studied compounds.
In the tetrapyrrolic macrocyclic systems considered in this study the five-membered heterocycle with two nitrogen and one chalcogen atom (sulfur or selenium) is annulated to the p-pyrrole carbons of one of the pyrrole rings. Molecules of five-membered heterocyclic compounds like pyrrole (C4NH5), furane (C4OH4), thiophene (C4SH4), selenophene (C4SeH4) and their azasubstituted analogues form the 6n-electron aromatic systems obeying the Huckel rule (4N+2, N=1), since the imine nitrogen atom, atoms of oxygen, sulfur or selenium deliver to this system pair of electrons from the pz orbital. As a result, annulation of these heterocycles is similar to annulation of the aromatic six-membered benzene
Ph
Ph
Ph Ph
Ph Ph
isomer b
H2{XN2}PAPh6 X = S, Se
*Bu
u
Bu
H2{XN2}PA(BZ'BU)4 X = S, Se
Chart 1. Structural formulae of porphyrazines with annulated chalcogen containing heterocycle.
rings leading to extension of the conjugated n-system. At the same time, the electronic structure and photophysics of the each compound has peculiarities determined by distribution of the electron density in the ground and excited states of the molecule.
Since our goal is the revealing of the nature of spectral changes upon annulation of heterocycle, we analyse and compare the electronic structure and spectral features in the selected pairs of molecules. The results of spectral luminescence study obtained for H2{SN2}-PAPh6 and H2{SeN2}PAPh6 are compared with the data on octaphenylporphyrazine (H2PAPh8), and results for H2{SeN2}PA(BziBu)3 - with the data for the corresponding phthalocyanine, H2I3A(BziBu)4 (H2Pc = H2PABz4). For deeper interpretation of the experimental results we have also made quantum-chemical calculations of the ground and excited states of H2PAPhg H2{SN2}PAPh6, H2Pc and H2{SN2}PABz3.
Materials and Methods
Synthesis of H2{SN}PAPh6 and H2{SeN}PAPh6 was conducted as described earlier.[1] During chromatographic isolation of these compounds the symmetrical porphyrazine H2PAPh8 was obtained as by-product. Its spectral properties are identical with the product obtained following the original Linstead procedure.®
Synthesis of H2{SeN}PA(BzfBu)3 is similar to that of the S-analogue H2{SN2}PA(Bz'Bu)3 described earlier.[3] Template condensation of 1,2,5-selenadiazole-3,4-dicarbodinitrile (0.09 g, 0.5 mmol) and 4-terf-butylphthalodinitrile (0.28 g, 1.5 mmol) was conducted in a solution of magnesium butoxide (from 0.1 g, 4 mmol of magnesium) in dry n-butanol under reflux for 5 h. After evaporation of the solvent the residue was washed with 50% aqueous acetic acid (10 ml), then with water, dried, dissolved in CH2Cl2 and chromatographed on alumina. The Mg11 complex Mg{SeN2}PA(Bz'Bu)3 which was isolated from the second fraction (MS (Maldi-TOF): m/z = 760; calc. for [M+] - 760.2) was demetallated in a CH2Cl2-CF3COOH mixture (1:1) overnight, and, after evaporation of solvents, the residue was chromatographed on alumina to give 9 mg of H2 {SeN2}PA(Bz'Bu)3 (yield ca. 3 %). UV-vis (CH2Cl2) Xmax nm (lge): 597 (4.03), 640 (43.46), 661 (4.43), 708 (4.79). IR (KBr)v cm-1: 3305 (vNH), 2958s, 2904m, 2869m, 1616m, 1533w, 1483m, 1394m, 1364m, 1315m, 1257m, 1186w, 1090m, 1010s, 914w, 893w, 832m, 745s, 675m, 617w, 518w, 546w. 1H
NMR (CD2Cl2) 8 ppm: -2.67 (2H, s, NH), 1.26-1.38 (27H, s, tert-Bu), 7.50-9.25 (9H, m, CH ).
\ ^ 7 arom7
The spectral-luminescent characteristics were measured using the automated spectrometric apparatus, described in details elsewhere.[4] The fluorescence quantum yield (®F) was measured relative to a solution of tetraphenylporphine in toluene for which ®F = 0.09 (in the presence of air oxygen).[5] The absorption spectra were recorded on a Varian Cary-500 Scan spectrophotometer. Toluene was used as a solvent at ambient temperature, and for the low-temperature measurements at 77 K we have used the vitrifying mixture diethyl ether - toluene (2:1), abbreviated as ET.
Quantum-chemical calculations of the geometric structure were performed by unrestricted Harthree-Fock method using the AMI Hamiltonian and MOPAC 6 package.[6] Calculations of the excited electronic states of the molecules were made by INDO/ Sm method[7] (modified INDO/S method) using our own program. It should be noted that this new INDO/Sm parametrization was elabotated by us recently specially for calculations of porphyrin type systems. The results of the calculation of the Q transitions for the fundamental porphyrin systems such as chlorin, bacteriochlorin, tetrabenzoporphine, tetraazaporphine with this INDO/Sm method differ from the experimental data by ~ 300-700 cm-1, while the error of the standard INDO/S method is not less than 3000 cm-1. Calculations of the electronic states have taken into account ca. 400 singly excited electronic configurations with use of the molecular orbitals obtained in the INDO/Sm procedure.
Spectral-Luminescent Properties
H2PAPh8. The electronic absorption spectrum of H2PAPh8 is shown in Figure 1A (curve 1). Comparison with the spectrum of unsubstituted porphyrazine (tetraazapor-phine), H2PA = H 2TAP, in chlorobenzene (see ref. [8]) shows, that attachment of eight phenyl groups does not change the general spectral pattern, but leads to considerable bathochro-mic shift of the long-wave absorption bands Qj(0-0) and Q2(0-0): from ~617 to ~668 nm (i.e. by ~51 nm or ~1200 cm-1) and from 545 nm to 603 nm (i.e. by ~58 nm or ~1800 cm-1), respectively. The Soret band (exactly speaking, its analogue) is shifted bathochromically even stronger: in the X scale from 333 nm to 369 nm, i.e. by 36 nm or 2900 cm-1. One should also note the decrease of the energy gap Q2-Ql (AEqq ) from 2140 to 1610 cm-1, the growth of the intensity of the2 d1 iffuse absorption band on the long-wave side of the
Soret band and the broadening of the visible bands, especially of the ^(0-0) band. This broadening, is connected, most likely, with fluctuations and torsion movements of phenyl rings, which leads to the disturbance of the system and activation of the low frequency non-planar vibrations in the electronic spectra. The observed bathochromic shift results from destabilisation of the highest occupied a1u-type orbital (notation for the D4h symmetry) due to electron-donating effect of phenyl groups.
The fluorescence spectrum of H2PAPh8 is also shown in Figure 1A (curve 2). It can be seen that mirror symmetry of
1.00.80.60.40.2-
O.O
300 400 500 600 700 800
900
300 400 500 600 700 800 900
Figure 1. Absorption (1) and fluorescence (2) spectra of H2PAPh8 (A, Xexc=602 nm), H2{SN2}PAPh6 (B, Xexc=577 nm) and H2{SeN2}-PAPh6 (C, Xexc=574 nm) in toluene at 293 K.
the absorption and emission spectra is absent, which can be explained by overlap of the Q2(0-0) band with the vibrational structure of the transition. The Stokes shift is
relatively large (~13 nm, or ~290 cm-1), and halfwidth of the 0-0-band in fluorescence is larger than of the Q1(0-0) band. These facts are indicative of structural changes (very likely with appearance of non-planarity) in the singlet excited state ^1(Q1). The spectral shifts due to octaphenyl substitution (transition from H2PA to H2PAPh8) are comparable, and in the UV region even stronger, than due to extension of the n-conjugated system in the case of symmetrical annulation of four benzene rings, i.e. in going from H2PA to H2Pc (see [9]). However for H2Pc, its metal complexes and their close analogues the Q bands are narrower and the Stokes shift is very small (~50 cm-1), which are considered as an evidence of the planarity of the molecules.
H2{SN2}PAPh6 and H2{SeN2}PAPh6. The absorption spectrum of H2{SN2}PAPh6 is shown in Figure 1 B (curve 1), together with the fluorescence spectrum (curve 2). Similar data for H2{SeN2}PAPh6 are displayed in Figure 1 C. The obtained data evidence that substitution of two phenyl rings in the molecule of H2PAPh8 by five-membered het-erocycle, 1,2,5-thiadiazole or 1,2,5 selenadiazole, leads to
300 400 500 600 700 800 900
A, nm
Figure 2. Absorption (A, curve 1) and fluorescence spectra (A curve 2, B, Xexc=350 nm) of H2SeN2(Bz'Bu)3 in toluene (A; B, curve 1) at 293 K and in the mixture diethyl ether-toluene (2:1) at 77 K (B, curve 3). Curves A2 and B2 are identical but have different scale on the X axis.
the bathochromic shift of the absorption band Qj(0-0) by 16 and 25 nm (350 cm-1 and 540 cm-1), respectively, and also to considerable growth of the energy gap Q2-Q1 from
hEn = 1610 cm-1 to AE„„ = 2700 cm-1 (S) and 3000 cm-1
Q2Q1 q2q1 (Se). The fluorescence spectra are shifted respectively, the
Stokes shift and half-width of the 0-0-bands of fluorescence and absorption are somewhat decreased, which might be indicative of increase in the rigidity of the macrocycle. Due to increase of the Q2-Qj gap the spectra of fluorescence and absorption become almost mirror-symmetrical. The fluorescence quantum yield in both cases is the same (Op = 0.06).
H2{SeN2}PA(Bz'Bu)3. In contrast with the; H2PAPh derivatives, in the absorption spectra of H2{SeN2}PA(BztBu)3 (Figure 2, A, curve 1) the Qj(0-0) is relatively narrow (halfwidth 16 nm, which corresponds the data for H2PA(Bz'Bu)4[10]). At the same time the 0-0-band of the fluorescence spectrum (Figure 2, A,B, curve 2) is slightly broader (halfwidth 20 nm) and the Stokes shift is 18 nm. The latter might evidence about some structural change of the molecule (together with solvate surrounding) in the Q1 state. At 77 K the fluorescence bands (Figure 2, B, curve 3) as well as absorption bands become narrower and their vibrational structure is better seen. The comparison of the absorption spectrum of H2{SeN2}-PA(BztBu)3 (Figure 2, A) with the absorption spectrum of H2PA(Bz'Bu)4[10] shows that substitution of one benzene ring by 1,2,5-selenadiazole moiety leads to a bathochromic shift of the Qj(0-0) band by ~10 nm. Considerable changes are also observed for H2{SeN2}PA(BztBu)3 in the region of the second electronic transition. As a result the Q2(0-0) band is broadened, its intensity is decreased, and the Q2-Q1 gap is increased to AE„ n = 1620 cm-1 from AE„ n = 900 cm-1 for RPA(Bz'Bu),.
Q2Q1 Q2Q1 2 4
These peculiarities of the Q2 band are evidently connected with action of the vibronic analogue of the Fermi resonance (see ref. [1112]). In the absorption spectrum of the S-analogue H2{SN2}PA(BztBu)3 in dichloromethane the maxima of the Q1 and Q2 transitions were observed at 693 and 657 nm with AE = 790 cm-1.[3] Therefore upon substitution of S by Se the Qj ti"ansition is shifted bathochromically by 17 nm (330 cm-1), while the Q2 transition undergoes a hypsochromical shift by 20 nm (480 cm-1).
The fluorescence quantum yield for H2{SeN2}-PA(BztBu)3 OF = 0.14, while for H2PA(BztBu)4 OF = 0.77. [13] The quenching of the fluorescence might be due to the internal effect of the heavy atom of Se (see ref. [14,15]). The fluorescence quantum yield for the derivatives of H2PAPh8 is lower (OF = 0.06), but for H2PAPh8 it is even lower, i.e. fluorescence is not quenched, but enhanced upon annulation of heterocycle. The reason of the weak fluorescence of H2PAPh8 is not clear. It should be noted that the equal values of OF for compounds containing atoms of sulfur (atomic number Z = 16) and selenium (Z = 34) might be explained, at least partly, by the compensation of the heavy atom effect by decrease of the conjugation.
orbitals p.; their delocalization along the molecular fragments l. = Z „ C 2, where C - the coefficients of the atomic orbital,
A ueA jUj 7 jUj
A - the number of fragment; characteristics of the Sk states themselves including transition energies from the ground state (E), transition oscillator strengths f), angle between the transition moment and the x axis, directed along the NH-HN axis, the degree of the electronic density transfer from one fragment to another (LAB), and degree of the localization of excitation on the fragments (LAA) (see ref. [1617]). For H2Pc and H2{SN2}PABz3 the following fragments have been chosen: 1 - internal 16-membered ring (azacyclopolyene), 2 - all benzene rings together with the Cb-Cb bonds (b indicates p-positions of the pyrrole ring), 3 - annulated heterocycle. In the case of H2{SN2}PAPh6 and H2PAPh8 the Cb-Cb bonds of the pyrrolic rings which have attached phenyl groups are included in the fragment 1, and fragment 2 includes all phenyl rings. The calculations performed with another atomic distribution, which includes all Cb atoms to the macrocyclic fragment lead to the similar results and are not reported in this paper. Results of calculations are displayed in Figures 4 (MO energies) and 5 (absorption spectra presented as so called "stick diagrams") and in Tables 1-3. Data for H2PA obtained previously[7] are also presented for comparison.
Geometric Structure of the Ground State
Geometric optimization has shown that the molecule of H2PAPh8 has almost planar structure of the macrocycle, and the dihedral angles yn between the plane of the phenyl ring n and the plane of the macrocycle are ~40°. Annulation of the 1,2,5-thiadiazole ring instead of two phenyl rings (i.e. transition to H2{SN2}PAPh6) has only a slight influence on the dihedral angles yn. Figure 3 shows the optimized structures of H2PAPh8 (A) and" H2{SN2}PAPh6 (B) in two projections. For H2Pc and H2{SN2}PABz3 the geometry optimization leads to the planar structure of the molecule. It should be noted that compouds containing one annulated heterocycle can exist as two different NH-isomers (tautomers) - a and b. In isomers a the axis NH-HN goes through pyrrole rings without annulated heterocycle, and in isomers b - through pyrrole rings, one of which contains annulated heterocycle. The ground state energy for the isomer a is considerably lower than for the isomer b for the both investigated compounds H2{SN2}PABz3 and H2{SN2}PAPh6. Similar behaviour was noticed previously[18] for derivatives of porphine (H2P) and H2PA with one annulated 5-membered heterocycle or benzene rings. In the case of H2{SN2}PAPh6 the energy of the isomer b is higher than for the isomer a by 0.25 eV (~2000 cm-1 or 6 kcal/mol), and in the case of H2{SN2}PABz3 by 0.18 eV (~1500 cm-1 or 4 kcal/mol). Since calculations predict the practical absence of the NH-tautomery in the ground state for H2{SN2}PAPh6 and H2{SN2}PABz3, in the following discussion the results are shown only for the isomers a.
Quantum-Chemical Calculations
Calculated Characteristics of the Excited States
The following characteristics have been determined in calculations of excited singlet states Sk: energies of molecular
Properties of Molecular Orbitals in H2PA and Its Derivatives
In the case of H2PA the highest occupied MO (HOMO) 9-1 is similar in its structure to the HOMO in the metal complex MPA which has a1u symmetry of the D4h group. It is also similar with one of the quasi-degenerated HOMO (1a1u and
B
Figure 3. Optimized structures of H2PAPh8 (A) and isomer a for H2SN2PAPh6 (B).
3a ) of the four-orbital model that adequately describes the for all considered cases is not lower than C and n—n*
lowest excited singlet states in metal porphyrins MP and freebase porphyrins H2P (symmetry groups DAh and D2h) (see e.g. [919]). In the four-orbital model, mixing of the aueg and a2ueg configurations gives two low-energy excited states Qx and Q2, the transitions to which from the ground state G have low intensity and determine the absorption spectra of porphyrins in the visible region, and states B1 and B2, corresponding to the high intensity Soret band near 400 nm. Here eg - is the doubly degenerate lowest unoccupied MO (LUMO). The N and N2 states, transitions to which have intermediate intensity, are located higher. The second HOMO, <p-2, of the four orbital model is 3a2u, which upon meso-azasubstitution (transition from H2P to H2PA), becomes lower in energy and mixes with other MO. Because of this and due to changes in the configuration composition the transitions G—Bt and G—^B2 become less intensive, while transitions G—Nt h G—N2 are intensified. Nevertheless we, as well as the other authors name the intense absorption band in H2PA and MPA near 330 nm the Soret band (or its analogue).
In the following discussion for notation of the Q, B and N states it is better to use indexes x and y instead of 1 and 2, since the symmetry of the macrocyclic conjugated system
transitions should be polarized along the axis x (NH-HN) and y (perpendicular to x in the plane of the molecule). As well as for most derivatives of H2P, the quantum-chemical calculations for H2PA give the polarization of the G^Qj transition along the x axis, i.e. Qx = Qx, and Q2 = Qy.
As can be seen from Figure 4 and Table 1 the introduction of eight phenyl groups increases the energy of the HOMO 9-1 «a1u» («» indicates the absence of strict D4h symmetry), while the energy of two LUMOs is only slightly increased. This explains the observed bathochromic shift of the Q bands in H2PAPh8 as compared to H2PA. Figure 4 shows also the energy levels of the lower lying filled MO 9-2...<p-5. It can be seen that upon octaphenyl substitution all of them are destabilized and became closer to each other. Apparently this occurs due to conjugation of the phenyl groups with macrocycle (antibonding). This effect should be responsible for the observed bathochromic shift of the Soret band in the experimental spectra in going from H2PA to H2PAPh8.
Annulation of four benzene rings in going from H2PA to H2Pc leads to similar changes in the MO level diagram, but the energy of the HOMO «a1u» is more considerably increased as well as that of the vacant levels ^ and <p2.
Table 1. Energies g. of the orbitals [a] and MO localization on the molecular fragments l
i g., eV h l2 l3 g., eV l1 l2 l3
3 h2pa -1.12 H2PAPh8 (Yn» 40°) 72 28 -0.92 H2PAPh8 (y = 60°) 2 88 1 19
2 -2.33 -2.32 90 10 -2.25 94 6
1 -2.43 -2.37 90 10 -2.31 94 6
-1 -6.16 -5.95 90 10 -5.95 93 7
-2 -7.89 -6.77 44 56 -7.07 51 49
-3 -8.34 -6.82 47 53 -7.10 53 47
-4 -8.59 -6.96 50 50 -7.20 61 40
-5 -7.20 34 66 -7.50 37 64
-6 -8.10 55 45 -8.01 24 76
-7 -8.17 21 79 -8.04 21 79
-8 -8.19 16 84 -8.17 58 42
6 H2{SN2}PAPh6 (y„ » 40°) -0.17 32 67 1 H2{SN2}PAPh6 (y = 60°)
3 -1.52 25 3 73 -1.50 26 2 73
2 -2.37 86 6 8 -2.34 88 4 8
1 -2.63 87 8 5 -2.58 90 5 6
-1 -6.07 87 8 5 -6.08 90 5 5
-2 -6.88 45 55 0 -7.19 51 49 0
-3 -7.04 47 53 0 -7.32 56 44 0
-4 -7.27 33 66 0 -7.59 35 65 0
-5 -8.24 34 66 0 -8.11 21 79 0
-6 -8.27 20 80 0 -8.32 55 45 0
3 h2pa H2Pc -02.61 41 59 -1.38 H2{SN2}PABz3 2 227 3 9 65
2 -2.33 -2.01 70 30 -2.29 69 23 8
1 -2.43 -2.20 71 29 -2.42 74 19 7
-1 -6.16 -5.38 72 28 -5.64 74 21 6
-2 -7.89 -7.76 63 37 -7.99 60 40 0
-3 -8.34 -7.90 28 72 -8.17 25 75 0
-4 -8.59 -8.04 18 82 -8.57 10 89 1
[a] Numeration of 9.: negative i values correspond to the filled MOs and positive to the vacant ones in the ground state.
[b] The following molecular fragments are chosen: 1 - internal 16-membered macrocycle (azacyclopolyene) for H2Pc and H2{SN2}PABz3; 24-atomic porphyrazine macrocycle for H2PAPh8 and H2{SN2}PAPh6; 2 - all benzene rings together with the Cb-Cb bonds for H2Pc and H2{SN2}PABz3 or all phenyl rings for H2PAPh8 and H2{SN2}PAPh6; 3! - annulated heterocycle for H2{SN2}PAPh6 and H2{SN2}PABz3.
E, eV 0
-2H ■A
-6H -8
-10H
"O 0) Q.
u o o
— O
T3 £ "q.
o o O
Figure 4. Energies of the highest occupied (HOMO) and lowest unoccupied molecular orbitals (LUMO) for H2PA and its derivatives: 1 - H2PA, 2 - H2PAPh8 (yn « 40°) 3 - H2PAPh8 (Y = 60°), 4 - H2{SN2}PAPh6 (Yn « 40°), 51 - H2{SN2}PAPh6 (Y = 60°), 6 - H2Pc, 7 - H2{SN2}PABz3.
The energy levels of the other filled MOs are destabilized and draw together to a lesser extent than in the case of H2PAPh8. Upon annulation of the 1,2,5-thiadiazole ring (instead of the benzene ring or two phenyl groups) the energy of the HOMO «a » is only slightly changed. The changes in the MO 9-2.. ,<p_5 is stronger - the effect of their rapprochement mentioned above weakens and their energy level distribution becomes closer to the initial level scheme for H2PA.
Electronic Absorption Spectra
The results of calculations are presented in Figure 5 and Tables 2 and 3. Figure 5 shows also the experimental absorption spectra of H2PAPh8, H2{SN2}PAPh6 and H2{SeN2}PA(BztBu)3 on the frequency scale.
H2PAPh8. For H2PAPhg the calculated energies of the G—Qx and G—Qy transitions (EQ = 14900 cm-1 and Eq = 16400 cm-1) and the energy difference between them
Qy
(AEQ = 1500 cm-1) are in a good agreement with the experimental values (15000 and 16600 cm-1, AEQ Q = 1600 cm-1).
M
H,PA
1.5 1.0 0.5
LO OO
OO <q CO CO CO CO
r-- 1 ^ CO C\J OO
10 1 CM
1 1
10
15
20
25
30
35
40
(7=40°)
(Y =60°)
H2{SN2}PAPh6 (Y =40°)
H2{SN2}PAPh6 (Y =60°)
H,Pc
H2{SN2}PABZ3
1.5 1.0 0.5
1.5 1.0 0.5
10
15
20
25
30
35 40
si <sr 00 0 CO ■«fr
1 10 CD CM S!
10 15 20 25 30 35 E,10"3cm"1
40
Figure 5. Electronic absorption spectra of H2PA and its derivatives calculated using the INDO/Sm method. For H2PAPh8 and H2{SN2}-PAPh6, and for H2{SeN2}PA(Bz'Bu)3 the experimental absorption spectra normalized to the maximum of the Q band are also shown.
The theoretical and experimental shifts in going from H2PA to H2PAPhg agree also quite well (Table 3).
In contrast to this the comparison of the calculated data on electronic transitions with the absorption bands of H2PAPhg in the blue and UV regions is difficult. Because of that we have made additional calculations assuming that the energy of the N states in the initial calculations was underestimated due to overestimation of the degree of conjugation between the phenyl groups and macrocycle. As a hypothesis we have assumed that for the H2PAPhg molecule in solutions due to intermolecular interactions the dihedral angle between planes of phenyl and macrocycle might be larger than in an isolated molecule (yn « 40°). The geometry was then optimized with the fixed values of the dihedral angles yn = y (i.e. all yn are equal), and also with rotation of the three selected phenyl groups in the different pyrrole rings, which can model the situation in the solution.
The calculations have shown that upon increase of the dihedral angle y all transitions are shifted hypsochromically
to a different extent. This shift is caused by the increase of the energy gap between LUMO and HOMO. The orbital «a1u», as well the 9 h <2 orbitals which have the electronic density localized on the porphyrazine macrocycle are less sensitive to changes of y values. Therefore the G^-Q transitions are also less sensitive to the changes of y. Upon increase of the phenyl rotation angle y the orbitals <-2 to <-5 are considerably lowered (for y = 60° the difference is « 0.3 eV). Approximately the half of the electron density of these orbitals resides on phenyl rings which determines their sensitivity to changes of the y values. Correspondingly, the energy of the G^-N transitions is changed more considerably when the phenyl rings are rotated. In all calculated variants the changes in the G^-N transition energies are approximately 4.5 times larger than for the G^Q transitions, which can explain the considerable broadening of the absorption bands in the blue and UV spectral regions. It should be noted that along with the increase of the y values, some of the MOs which are lower than <-5, increase their energy, which leads to the change of their relative position. These orbitals are less sensitive to the variation of y than the <-2-<-5 orbitals. As a result the change in the energy of the transitions near 30000 cm-1 with variation of y is less, than for the G^-N transitions, although it is 2.5 times larger, than for the G^-Q transitions.
However, the results of calculations with increased values of the dihedral angles y do not give an unambiguous correspondence between the calculated transition energies and experimental absorption bands in the 22000 - 33000 cm-1 region. Below we consider two possible assignments of the calculated and experimental spectra.
The first variant of the assignment is based on the direct correlation of the energies and oscillator strengths of the calculated transitions with allowance made for above mentioned factor of the band broadening in the blue and UV spectral regions. In this case the band at 451 nm can be correlated with four transitions two of which are of low intensity (at 22800 and 23100 cm-1), and the band at 370 nm (27030 cm-1) - with transitions of medium intensity at 30100 and 30300 cm-1. Following facts are in support of such assignment. (i) For H2PA the calculated energies of the N levels are also higher than in the experiment by ~3000 cm-1. (ii) The integral intensity of the broad band at 451 nm is not low. (iii) The broadening of the absorption bands in this region correlates with high sensitivity of the G^N transitions to rotation of phenyl groups. As was shown by calculations with variation of the dihedral angles yn, their energy varies from 22000 to 28000 cm-1.
However such assignment provokes some objections. (i) The 370 nm band is also broad and its intensity is considerably higher than that of the 451 nm band, i.e. most likely the first one is the analogue of the Soret band. (ii) Comparison of the experimental absorption spectra of H2PA and H2PAPh8 shows their similarity: introduction of eight phenyl groups leads to the bathochromic shift of all spectral bands. (iii) Configuration composition of the states with energy 30100 and 30300 cm-1 does not include the electronic configurations related to configurations of the four-orbital model which dominate in the electronic spectra of the H2P and H2PA derivatives.
The second variant of the assignment assumes that the average value of the dihedral angles y should be larger
Table 2. Characteristics of transitions into the lowest excited states.[a]
E-10-3, cm-1 f[b] Configuration composition of the excitation Lu L12 L13 L21 L22 L23 L31 L32 L33
h2pa
15.7 0.61 x 9.1^12-0.93)
17.8 0.81 y 9^(0.97)
26.6 0.61 x 9292(-0.85)
28.4 0.46 y 929(-0.92)
33.5 1.18 y 94^(0.92)
33.8 1.47 x 9-492(0.88)
H2PAPh8 (y « 40°)
14.9 0.58 x 9-19(-0.93) 79 9 10 1
16.4 0.71 y 9-19,(0.95) 77 9 12 2
20.6 0.25 y 9-291(0.89) 45 4 46 4
22.8 1.06 y 9-491(-0.88) 9-291(-0.32) 38 4 52 5
23.1 1.63 x 9-492(0.76) 9-292(0.49) 45 6 44 5
30.1 0.44 y 9792(0.72) 9-691(0.55) 38 4 51 7
30.3 0.55 x 9è92(-0.74) 9,19s(0.38) 51 17 28 4
H2PAPh8 (y = 60°)
15.4 0.65 x 9-1918-0.94) 87 6 7 1
17.1 0.81 y 9-192(-0.96) 85 6 7 1
22.9 0.12 y 9-291(0.91) 58 3 37 2
25.3 1.13 y 9-491(-0.88) 9-291(0.33) 44 3 50 3
25.7 1.91 x 9,492(0.79) 9-29/-0.50) 50 4 43 3
31.5 0.57 y 9-891(-0.91) 62 5 30 3
31.7 0.48 x 9-892(0.87) 42 11 44 3
H2{SN2}PAPh6 (y « 40°)
13.9 0.66 x 9-191(0.94) 73 7 7 8 1 1 4 0 0
16.8 0.45 y 9-192(0.91) 71 6 6 12 1 1 3 0 0
19.3 0.13 y [c] 9-291(0.89) 48 4 2 42 4 1 0 0 0
21.5 1.02 y 9_391(-0.85 ) 9-192(0.34) ^(-Q^) 37 4 2 49 4 2 1 0 0
23.3 1.67 x 9-392(-0.83) 9-292(-0.38) 39 4 4 45 4 4 0 0 0
30.6 0.51 y [c] 9_591(-0.63) 939/-0.43) 37 6 10 30 4 14 0 0 0
30.9 0.47 x [c] 9-592(0.53) 9-196(-0.38) 44 14 5 26 4 5 1 1 1
H2{SN2}PAPh6 (y = 60°)
14.3 0.66 x 9-19(-0.95) 77 4 9 5 0 0 4 0 0
17.4 0.65 y 9-192(0.95) 78 4 7 7 0 0 4 0 0
21.7 0.05 y [c] 9-291(0.87) 58 3 2 35 2 1 0 0 0
23.9 0.91 y 9-391(0.88) 9-192(-0.25 ) 9-291(0.25) 45 2 3 45 2 2 0 0 0
25.7 1.57 x 9-392(0.81) 9-292(0.33) 50 3 4 36 2 3 1 0 0
30.7 0.36 y 9-691(0.85 ) 9-393(-0.34) 54 3 13 26 2 2 0 0 0
31.6 0.34 x 9è92(-0.65) 9^(0.44) 34 4 8 41 3 2 3 0 5
H2Pc
14.1 0.85 x 9-192(0.96) 51 22 19 9
14.4 1.14 y 9-191(0.99) 49 22 20 9
26.8 0.51 x 9-291(0.92) 39 15 34 12
30.9 0.25 y 9-292(0.95) 39 16 33 13
31.4 1.75 x 9-491(-0.91) 9-291(0.26) 22 10 51 18
33.4 1.21 y 9-492(-0.89) 9-19(0.31) 24 8 51 17
H2{SN2}PABz3
13.3 0.69 x 9-192(-0.95) 46 14 14 12 4 3 3 1 1
14.7 1.01 y 9-191(0.98) 52 15 6 16 5 2 4 1 0
18.1 0.13 x 9-193(0.97) 23 9 44 6 3 11 2 1 3
26.7 0.37 x 9-291(-0.82) 9-391(0.49) 35 10 2 40 10 2 0 0 0
29.8 0.47 x 9-19,(0.84) 9-391(0.43) 19 49 1 13 14 1 1 3 0
30.0 1.34 x 9-391(-0.70) 9-197(0.53) 30 25 2 26 11 2 1 1 0
30.7 0.33 y [c] 9-292(-0.94) 37 12 4 34 10 2 0 0 0
32.4 0.79 y 9-392(0.93) 21 7 3 51 15 3 0 0 0
[a]E - transition energy, f - transition oscillator strength, degree of localization (L ) and charge transfer (L ) between molecular fragments upon transition into the excited state as defined in the remark to Table 1. Axis x is directed along the NH-HN axis. Axis y is directed perpendicular to the x axis in the plane of the molecule. The data are given for the states with largest values of f.
[b] Polarization of the transition along x or y axis is indicated.
[c] Values of angles between the x axis and transition moment differs from 0° or 90° in the range from 2° to 8°.
Table 3. Shifts of the Qx and Qy levels (AEQ , AEQ ) and changes in the Qy-Qx splitting (8(AEg g ) )[a] in the selected molecular pairs (cm-1).
Parameter calculation Y = 40° calculation Y = 60° experiment^ calculation experiment
MQx AEQy 5(AW H2PA - H2PAPh8 -890 -320 -1240 -1400 -650 -1770 -510 -330 -530 H2PA - H,Pc[c] -1640 -1780 -3380 -3200 -1740 -1420
MQx AEQy 5(AW H2PAPh8 - H2{SN2}PAPh6 -990 -1120 -350 (S) /-540 (Se) 410 250 750 (S) / 840 (Se) 1400 1370 1100 (S) /1380 (Se) H,Pc - H2{SN2}PABz3[d] -750 130 (S) /-180 (Se) 300 120 (S) / 550 (Se) 1050 -10 (S) / 730 (Se)
[a]
- = ^ - ^
5(AEqq) = AE - AE = (E - E ) - ( E - E ) = (E - E ) - ( E
[b] ^^x Y^x2 V1 1 2 X1 x2 Xl
Experimental shifts are calculated using the spectral data obtained for H2PAPh8, H2{SN2}PAPh6, H2{SeN2}PAPh6 and literature data for H2PA.[8] and H2Pc.[20]
[c] Experimental shifts are calculated using the spectral data reported for H2PA[8] and H2Pc.[20]
[d] Experimental shifts are calculated using the spectral data for tert-butyl substituted derivatives - H2PA(Bz'Bu)4,[10] H2{SN2}PA(Bz'Bu)3[3] and H2{SeN2}PA(Bz'Bu)3 (present work).
(by ca 20°) as compared to y « 40°, and also that the calculated oscillator strengths for the G^B transitions are strongly underestimated. The calculations have shown that better correspondence with the experiment in the blue and UV spectral regions is observed at y = 60° and y = 75°. At
Y = 75° the correlation in the transition energy in the Soret band region is better than at y = 60°, but the intensity of the G^B transitions in the blue region is very low. At such values of the dihedral angle y the correspondence for the Q bands becomes worse. In Tables 1-3 and in Figure 5 the calculation results are shown for completely optimized geometry and also for the dihedral angle y fixed at 60° for H2PAPh8 and H2(SN2>PAPh6.
Calculation at y = 60° predicts for H2PAPh8 the following sequence of levels: By (22900 cm-1), Ny (25300 cm-1) and Nx (25700 cm-1). It can be seen that the accordance for the G^By transition is very good (^max= 451 nm corresponds to vmax= 22170 cm-1), but the energy of the N levels is strongly underestimated. It should be noted that the calculated intensity of the G^B transitions is low at complete optimization and at all values of y. It should be admitted that such interpretation leads only to partial agreement with the experiment. This can be due to the unknown real values of y in
' n
solution.
H2{SN2}PAPh6. For this molecule the series of calculations was made with variation of the dihedral angles Y, since the analysis of the spectra in the blue and UV regions has faced with similar difficulties which were observed for H2PAPh8 (see above). Tables 1-3 and Figures 4 and 5 present the calculation results for the structure with completely optimized geometry and with the fixed angle y = 60° (similarly to H2PAPh8). As compared with H2PAPh8 all MO from <-5 to <5 are stabilized. For the electronic absorption spectrum in the visible and near UV regions it is important that the LUMO < is more considerably shifted. The energy gap between MO from <-2 to <-5 is increased. Beside this, despite the decrease of the phenyl groups number from eight to six due to annulation of heterocycle, their contribution in the orbitals from <-5 to <5 practically does not change (see Table 1). As a result the variation of the dihedral angles
Y and y leads to the similar calculated transition shifts for
H2{SN2}PAPh6 and H2PAPh8. The shifts of the MO levels for 92 are similar to the shifts observed upon annulation of one 1,2,5-thiadiazole ring to the H2PA molecule (transition from H2PA to H2{SN2}PA).[18] The calculated shifts of the G^Q transition energy in the pairs H2PAPhg - H2{SN2}-PAPh6 and H2PA - H2{SN2}PA, are approximately equal, since the configurational composition of the Qx and Qy states in all four molecules are the same (almost "pure" 9-191 and 9-192 configurations).
Calculation of the electronic absorption spectrum with full geometry optimization for H2{SN2}PAPh6 gives relatively good accordance with the experiment in the region of the G^Q transitions, which becomes better at y = 60° (see Table 3 and Figure 5). Thus, calculations with y = 60° predict almost pure 9-191 and 9-192 transitions at 14300 and 17400 cm-1 with AEQ = 3100 cm-1, which is in a good agreement with the experimental values for H2{SN2}PAPh6 (14600 and 17300 cm-1, AEQ &= 2700 cm-1) and H2{SeN2}6 PAPh (14400 and 17400 cm-1, AE, „ = 3000 cm-1).
6 QyQx
In contrast to that in the short-wave region we could not reach good agreement between calculation and experiment by variation of the dihedral angles y or by application of the two assignment approaches considered above for H2PAPh8. Calculations with rotation of the phenyl rings in each of three pyrrole fragments also fail to improve agreement with the experiment.
In the region of the G^B and G^N transitions the calculations of the excited states energies predict the lowering of the By and Ny levels for H2{SN2}PAPh6 as compared to H2PAPhg, while the energy of the Nx level remains practically unchanged. This was observed at dihedral angles yn, obtained in full geometry optimization and at all varied values of y. In the experimental absorption spectrum the analogue of the Soret band for H2{SN2}PAPh6 is shifted hypsochromically to 356 nm (28100 cm-1), the absorption in the blue region becomes weaker and two bands seen at 423 and 447 nm are also hypsochromically shifted.
The shift of the Soret band for H2{SN2}PAPh6 to higher energy by 1100 cm-1 in respect to H2PAPh8 observed in the experiment can be explained by the influence on its position of the transitions near 30000 cm-1, which are less sensitive
to variation of the dihedral angles yn in the case of H2{SN2}-PAPh6. This band under consideration for phenyl substituted derivatives of H2PA undoubtly has complicated character, and this question will require more detail investigation.
H2Pc. The INDOS/m calculations of the H2Pc molecule with AMI optimized geometry give the energy of the Qx transition at 14100 cm-1 and Q transition at 14390 cm-1 (AEQ = 290 cm-1). The mean value of these transitions (1425)0xcm-1) is in a good agreement with the experimental values (14430 and 15150 cm-1, mean value - 14790 cm-1 in dimethylsulfoxide[20]). However, since the absolute polarization of the G^QX transition in the H2Pc molecule is unknown, there are some difficulties in interpretation of the results. For the majority of porphyrins it can be considered as well established that the G^QX transition in the cases of the D2h or C2v symmetry is polarized along the NH-HN axis. In going from H2PA to H2Pc, along with the bathochromic shift, the Q-bands approach closer each other, and the intensity of the Q1 band becomes higher than of the Q2 band (see ref. [9,19]). It cannot be excluded that the transition G^Q2 «overrides» the G^Q1 transition in degree of energy lowering and becomes the longest wave-length one. However the analysis of our own data on consequitive transition from tetrabenzoporphine (H2TBP) to H2Pc by substitution of the methine CH bridges by N-atoms,[2122] the literature data on absorption spectra of tribenzoporphyrazine[23] and some other facts show, that the Q1 state of the H2Pc molecule most likely corresponds to the Q1 state of the H2TBP and H2PA molecules and is the Qx state. It means that our calculations underestimate the intensity of the G^Qx transition in respect to the G^Qy transition. This occurs, most likely, due to overestimated contribution of the 18-membered azacyclopolyene in the electronic structure of the H2Pc molecule. Shifts of the Q-levels in H2Pc in respect to H2PA, which are obtained at such assignment, are given in Table 3.
In the Soret band region the calculations of the H2Pc spectrum give the electronic transitions similar to the transitions of H2PA, but with different spectral shifts. The level Bx becomes higher by 200 cm-1, the level By - by 2500 cm-1, while the Nx level is lowered by 2400 cm-1 and the Ny level by 100 cm-1 (the N levels are no longer quasi-degenerate). It should be noted that in the absorption spectrum of H2PA(Bz'Bu)4 in «-heptane which is characterized by narrower absorption bands, two components can be seen in the Soret region: shoulder at 358 nm and maximum at 338.5
nm,
[10]
i.e. 27530 and 29840 cm-1 (AE = 1610 cm-1). For unsubstituted H2Pc the similar splitting of the Soret band in dimethylsulfoxide (and in dimethylacetamide) is even larger AE = 2250 cm-1.[20] In calculations we have obtained 31420 and 33380 cm-1 (AE = 1960 cm-1) - the correlation with the experiment is not worse that in the case of H2PA.[718] It is important that according to the spectra of magnetic circular dichroism[20] the polarization of the G^N1 transition is similar to that of the G^Q transition and polarization of the G^N2 transition is similar with the G^Q2 transition. This is in agreement to the results of our calculations.
It is interesting to compare the results of the INDO/ Sm calculations for H2Pc and H2PA with the data obtained by the currently popular method TDDFT (time-dependent density functional theory) (see e.g. [2425]). In the paper[24] the electronic spectra have been calculated for H2P, H2PA,
H2TBP and H 2Pc, only results for the G^Q transitions being presented for H2PA and H2TBP. The authors[25] have studied the influence of protonation on the electronic structure and electronic spectra of H2Pc, and non-protonated molecule H2Pc is considered as an initial structure. These two works report practically the identical data for the Q-states of H2Pc. The comparison of our INDO/Sm results for H2Pc with the TDDFT results[2425] shows similarity for the sequence of the Qx and Qy levels and configurational composition of these states, and also for the values of the enegy gap AEQ . At the same time, the values of the Qx and Qy energy lev/els in our calculations show better agreement with the position of the Q bands in solution (see above). Descrepancy with the gas-phase data[26] is almost equal in value but deviation is of the opposite sign - in our work the energy of the Qx and Qy transitions is underestimated by ca 500 and 1600 cm-1, while in the TDDFT calculations[24,25] - overestimated by 2000 and 800 cm-1, respectively. For H2PA the TDDFT method gives even worse agreement with the experiment: EQ = 19200 cm-1, Eq = 20370 cm-1, while in the experimental spectrum in chlorobenzene EQ = 16200 cm-1, EQ = 18350 cm-1.[8] Our results obtained for xH2PA by INDO/Sm method are in a suitable agreement: EQ = 15700 cm-1, EQ = 17800 cm-1.[7]
Between the Soret band and Q bands the INDO/Sm calculations predict two transitions (which we denote as Bx h By, see above). The TDDFT method gives lower energy and intensity of these transitions as compared with INDO/ Sm method. Very likely these two transitions correspond to the weak and diffuse absorption band observed in the spectra of H2Pc near 450 nm. The configuration composition of the B states and their sequence is practically the same in calculations by these two methods.
The calculation results of both methods for the Soret band predict two intense transitions (denoted by us as Nx and Ny) in a similar sequence. The difference is that by TDDFT method several less intensive transitions were obtained with energies close to Nx and Ny. In our calculations these transitions correspond to configurations <-2<2, <-3<2, but their intensity is lower. Both methods overestimate the energy of the N and N states, but the TDDFT calculations - in a lesser
xy
degree. The Soret band maximum of H2Pc in the gas-phase is located at 29400 cm-1,[26] according to TDDFT calculations the mean value of the G^Nx and G^N transition energy is 31000 cm-1,[24 25] in our INDO/Sm calculations - 32400 cm-1. Both methods overestimate the relative intensity of the G^Nx transition. Therefore we can conclude that calculations of the electronic spectra of H2Pc by the INDO/Sm and TDDFT methods are in a reasonable agreement with each other and with the experiment.
H2{SN2}PABz3. Due to the absence of parametrisation for the Se atom in the AM1 and INDO/S methods we have considered the 1,2,5-thiadiazole derivative and accomplished calculations for the species without feri-butyl groups H2{SN2}PABz3. The calculated spectral parameters can be compared with the experimental spectrum of feri-butyl substituted species H2{SN2}PA(Bz'Bu)3 reported earlier[3] and with the spectrum of the Se-analogue H2{SeN2}PA(Bz'Bu)3 investigated in this work.
Substitution of one benzene ring by 1,2,5-thiadiazole (transition from H2Pc to H2{SN2}PABz3) leads to lowering of all orbitals from <-5 to <5. For all orbitals from <-2 to <2
this stabilization is approximately the same (deviations from the mean value of -0.25 eV do not exceed 0.03 eV). Earlier similar values of these changes have been obtained for the pair monobenzoporphyrazine (H2MBPA) - mono(1,2,5-thiadiazolo)porphyrazine (H2{SN2}PA).[18]
Calculations of the electronic absorption spectrum of H2{SN2}PABz3 give the series of levels comparable with the level scheme for H2Pc. The shifts of levels are from 100 cm-1 to 1400 cm-1. Calculation correctly predicts the direction of the shifts for the Qx and Qy energy levels and increase of the Qy-Qx gap (see Table 3). Calculated energy changes for Q-transitions when one benzene ring in H2Pc is exchanged by 1,2,5-thiadiazole ring in H2{SN2}PABz3 are close to that obtained for the pair H2MBPA - H2{SN2}PA.[18] According to calculations the Qx and Qy transitions are almost pure (9-192 and p-1p1 configurations, respectively). Their calculated
compounds with annulated heterocycle the calculation predicts a strong shift of the Qx band to the near IR region. This is mainly due to energy lowering of the one component of the strongly split quasi-degenerated LUMO «e » (Figure 4). This is connected, in turn, with conjugation of the five-membered cycle with 18-membered azacyclopolyene, which is transfomed to the branched 21-membered cyclopolyene.
Conclusions
The study of the influence of the 1,2,5-thia/ selenadiazole annulation on the spectral-luminescent properties of H2PJAPh8 and H2PA(BztBu)4 has revealed the correspondence between the initial and modified structures and some peculiarities. Annulation of the five-membered heterocycle leads to a bathochromic shift of the Qx band
energy (Eq = 13300 and Eq = 14700 cm-1) is somewhat lower and the Soret band, but the difference AE„ n is increased
Qx Qy ' '
and the predicted splitting (AEq = 1400 cm-1) is larger
QyQx
than in the experimental spectrum of H2{SN2}PA(BztBu)3 (Eq = 14400, Eq = 15200 cm-1 and AEq q = 800 cm-1[3]). It can*be seen fromyTable 3 that the effect of substitution of one of four benzene rings in phthalocyanine by 1,2,5-thiadiazole ring on the position of the Q transitions is poorly described by calculations. The calculations results agree better with the experiment for H2{SeN2}PA(BztBu)3 (see Figure 5), although the predicted bathochromic shifts are overestimated.
The levels of the states, which can be put in correspondence with the Nx and Ny states of H2Pc, are lowered by 1400 and 1000 cm-1, respectively. Taking into account such shift, the maximum of the Soret band observed for H2Pc at 333 nm,[20] should be observed for H2{SN2}PABz3 at 3347 nm. In the experimental spectra the Soret band maximum is observed at 353 nm for H2{SN2}(BztBu)3[3] and at 349 nm for H2{SeN2}(BztBu)3 (see F2gure 2, A).
Calculations show that annulation of the 1,2,5-thiadiazole ring leads to the appearance of new electronic transitions into the states with the predominant contribution of the configurations 9-193 at 18100 cm-1 and 9-197 at 30000 cm-1 (Table 2). These transitions possess characteristic feature of the intramolecular charge transfer (IMCT): the first one contains a large contribution of the electronic density transfer from the macrocycle to five-membered heterocycle, and the second one, from the macrocycle to benzene rings. In the experimental absorption spectrum of H2(SN2}(Bz'Bu)3 the first of these transitions ma be hidden by the Qy band and its vibronic satellites, but very likely it can correspond to the additional band which is seen in the spectrum of the Se-analog H2{SeN2}(BztBu)3 at 661 nm (15100 cm-1).
One can not exclude that in the solution the level of IMCT can be lowered due to fluctuations of solvation interactions, which opens an additional channel for nonradiation deactivation of the St state. This can explain, along with the heavy atom effect, the decrease of the fluorescence quantum yield for H2{SeN2}(BztBu)3 as compared to H2Pc. It should be noted that in the case of H2{SN2}PAPh6 the calculations do not predict the intramolecular charge transfer between the thiadiazole ring and macrocycle in the visible region.
Isomers b. Turning back to the problem of NH-tautomery, we should note that for isomers b of the considered
QyQx
and fluorescence spectra become mirror-symmetrical to the absorption spectra in the region of the G^Qx transition. Substitution of one benzene ring in H2PA(BztBu)4 by 1,2,5-selenadiazole leads to decrease of the fluorescence quantum yield from 0.77 to 0.14, which might be explained by an internal heavy atom effect, and also by contribution of transitions with intramolecular charge transfer. The fluorescence quantum yield for the H2PAPh8 derivatives is lower (0.06), but for H2PAPh8 it is even less, i.e. in this case fluorescence is not quenched but enhanced upon heterocycle annulation. The reason of very weak fluorescence of H2PAPh8 is not evident.
Quantum-chemical calculationsoftheground andexcited electronic states of the basis structures of phthalocyanine and octaphenylporphyrazine and their derivatives with annulated 1,2,5-thiadiazole ring (isomers a) has allowed an argumented interpretation of the experimental absorption spectra. Results obtained with the modified parametrization of the INDO/S method - INDO/Sm[7] describe the properties of the low-lying Q-states on a semiquantitative level. Effect of 1,2,5-thia/ selenadiazole annulation on the energies of the Q transitions is reasonably well reproduced for p-phenyl substituted porphyrazines and poorly for benzoannulated species. At the same time in the Soret band region of the phenyl substituted compounds (B and N transitions) the agreement is less adequate. In particular, the energies of the G^B and G^N transitions and ratio of their intensities depend strongly on the dihedral angle between phenyl rings and porphyrazine macrocycle. It is shown that the present halcogen-containing compounds considered exist in the form of the isomer a with the NH-HN axis passing through the pyrrole rings without fused heterocycle.
Acknowledgements. This study was partly financed by Belarusian Republican Foundation for Fundamental Research (Project F06R-141) and Russian Foundation for Basic Research (Project № 06-03-81022).
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Received 23.07.2009 Accepted 08.02.2010 First published on the web 22.03.2010
